Overlapping Spectrum In Optical Communication

ABSTRACT

An optical add-drop multiplexer including a first filter filtering a first band of wavelengths of a communication spectrum for a first communication segment and a second filter filtering a second band of wavelengths of the communication spectrum for a second communication segment. The second band of wavelengths overlaps the first band of wavelengths in an overlap band of wavelengths. The overlap band may have a variable size. The first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction the overlap band of wavelengths for the second communication segment.

CROSS REFERENCE TO RELATED APPLICATIONS

This U.S. patent application is a continuation-in-part of, and claims priority under 35 U.S.C. §120 from, U.S. patent application Ser. No. 14/151,938, filed on Jan. 10, 2014, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to overlapping spectrum in optical communication.

BACKGROUND

Initially introduced to connect two points across the ocean, submarine cable networks became more flexible with the introduction of optical add-drop multiplexers (OADM). OADMs used in optical communication networks are capable of removing wavelength channels from multiple wavelength signals and adding channels to those signals. Conventional OADMs have typically been limited to use in a relatively few nodes within a network, because of their inherent performance characteristics. In other words, as the number of conventional OADMs increases within the network, the limitations associated with conventional OADMs substantially affects network performance. Moreover, conventional fixed OADMs have two limitations (i) loss of spectrum due to a guard band that needs to be allocated between bands and (ii) add/drop ratio has to be determined at the time of system deployment and that stays fixed for the life of the submarine cable (˜25-30 years) as it is not practical to extract the OADM from the submerge location on the ocean floor and change the add/drop ratio.

SUMMARY

The present disclosure addresses the limitations of conventional OADMs without the drawbacks of ROADMs. One mechanism for overcoming the fixed/drop ratio is the use of reconfigurable optical add/drop multiplexers (ROADMs) that incorporate a dynamic wavelength switching element. The main downsides of a ROADM in sub-sea networks are: (i) reliability concerns of switching elements; (ii) the need for a reliable control channel to trigger a change in ratio; and (iii) the guard band disadvantage is still present even with ROADMs.

One aspect of the disclosure provides an optical add-drop multiplexer that includes a first filter filtering a first band of wavelengths of a communication spectrum for a first communication segment and a second filter filtering a second band of wavelengths of the communication spectrum for a second communication segment. The second band of wavelengths overlaps the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band. The overlap band may have a variable size. The first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment.

Implementations of the disclosure may include one or more of the following features. In some implementations, the first band of wavelengths includes the entire overlap band of wavelengths for the first communication segment and the second band of wavelengths excludes the overlap band of wavelengths from the second communication segment.

The overlap band of wavelengths may include common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. In some examples, the first filter and/or the second filter provide a fixed sized overlap band of wavelengths of the communication spectrum. In other examples, the first filter and/or the second filter are tunable or adjustable to provide a variable sized overlap band of wavelengths of the communication spectrum. The filtering may be used for adding or dropping wavelengths. At the add/drop location, the dropped wavelengths from one segment can be re-used in the next segment.

Another aspect of the disclosure provides an optical system that includes a first trunk terminal, a second trunk terminal, and a communication trunk coupling the first trunk terminal to the second trunk terminal. The system also includes a branching unit disposed along the communication trunk and coupling a branch terminal to the communication trunk. The branching unit includes an optical add-drop multiplexer having first and second filters. The first filter filters a first band of wavelengths of a communication spectrum for a first communication segment, and the second filter filters a second band of wavelengths of the communication spectrum for a second communication segment. The second band of wavelengths overlaps the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band. The first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment.

In some implementations, the first band of wavelengths includes the entire overlap band of wavelengths for the first communication segment and the second band of wavelengths excludes the overlap band of wavelengths from the second communication segment. The overlap band of wavelengths may be reserved for communications between the first and second trunk terminals.

The overlap band of wavelengths may include common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. In some examples, the first filter and/or the second filter provide a fixed sized overlap band of wavelengths of the communication spectrum. In other examples, the first filter and/or the second filter are tunable or adjustable to provide a variable sized overlap band of wavelengths of the communication spectrum. The filtering may be used for adding or dropping wavelengths. At the add/drop location, the dropped wavelengths from one segment can be re-used in the next segment.

In some implementations, at least one of the first trunk terminal, the second trunk terminal, and the branching unit controls a size of the overlap band. In other implementations, the optical system includes a spectrum manager in communication with the first trunk terminal, the second trunk terminal, and the branching unit. The spectrum manager controls a size of the overlap band. In some examples, the spectrum manager assigns a portion of the overlap band to wavelengths of the first communication segment and a remaining portion of the overlap band to the second communication segment.

Yet another aspect of the disclosure provides a method of optical communication that includes filtering a first band of wavelengths of a communication spectrum for a first communication segment and filtering a second band of wavelengths of the communication spectrum for a second communication segment. The second band of wavelengths overlaps the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band, the overlap band having a variable size. The first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment.

In some implementations, the method includes altering a size of the overlap band. The first band of wavelengths may include the entire overlap band of wavelengths for the first communication segment and the second band of wavelengths may exclude the overlap band of wavelengths from the second communication segment. Other delegations of the overlap band are possible as well.

The overlap band of wavelengths may include common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. The method may include assigning a portion of the overlap band to wavelengths of the first communication segment (e.g., express path) and a remaining portion of the overlap band to the second communication segment (e.g., add/drop path). In some examples, the first filter and/or the second filter provide a fixed sized overlap band of wavelengths of the communication spectrum. In other examples, the first filter and/or the second filter are tunable or adjustable to provide a variable sized overlap band of wavelengths of the communication spectrum. The filtering may be used for adding or dropping wavelengths. At the add/drop location, the dropped wavelengths from one segment can be re-used in the next segment.

In some implementations, the method includes receiving the first communication segment from a first trunk terminal, where the first communication segment is destined for a second trunk terminal coupled by a communication trunk with the first trunk terminal. The method also includes receiving the second communication segment from a branch terminal coupled to the communication trunk. The overlap band of wavelengths may be reserved for communications between the first and second trunk terminals.

The details of one or more implementations of the disclosure are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an exemplary optical communication system.

FIG. 2 is a schematic view of an exemplary transmittance having a guard band removed to form an overlap band.

FIG. 3 is a schematic view of an exemplary transmittance having an overlap band of wavelengths dedicated to one of two communications forming the transmittance.

FIG. 4 is a schematic view of an exemplary optical communication system using C- and L-band wavelengths.

FIG. 5 is a schematic view of an exemplary transmittance having an overlap band of wavelengths split between two communications of the transmittance.

FIG. 6 is a schematic view of an exemplary arrangement of operations for a method of optical communication.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, an optical communication system 100 includes first and second trunk terminals 110, 120 (also referred to as stations) coupled to a communication trunk 102. The coupling may be any connection, link or the like by which signals carried by one system element are imparted to the “coupled” element. The coupled elements may not necessarily be directly connected to one another and may be separated by intermediate components or devices that may manipulate or modify the signals. The communication trunk 102 may include a plurality of optical cable segments 102, 102 a-n (e.g., optical submarine cables) that carry optical signals 105 on associated optical channels/wavelengths λ.

One or more branch terminals 130 may be coupled to the communication trunk 102 between the first and second trunk terminals 110, 120 by corresponding branching units 140. A branching unit 140 may be an OADM branching unit. Moreover, one or more repeaters 150 and linking optical cables 102 may couple the branch terminal 130 to its corresponding branching unit 140. The system 100 may therefore be configured to provide bi-directional or uni-directional communication of optical signals 105 between terminals 110, 120, 130.

Branching units 140 enable the function of capacity redirection between express paths (e.g., from Station A to Station C) and add/drop paths (e.g., from Station A to Station B and/or Station B to Station C). This can be done, for example, by simultaneously adding/dropping a band B of wavelengths λ at each OADM 140. The terms “add/drop,” “adding/dropping,” and “added/dropped” refer to either the operation of adding one or more wavelengths λ, dropping one or more wavelengths λ, or adding wavelengths λ and dropping others. Those terms are not intended to require both add and drop operations, but are also not intended to exclude add and drop operations. The terms are merely used as a convenient way to refer to either adding or dropping or both adding and dropping operations.

In general, the branching units 140 may add and drop channels λ to/from the communication trunk 102. In some implementations, a wavelength division multiplexing (WDM) signal 105 may originate at one or more of the terminals 110, 120, 130, and the branching units 140 may be configured either to pass some channels λ through the branching units 140 to travel uninterruptedly through the communication trunk 102 from an originating trunk terminal 110, 120 to a receiving trunk terminal 110, 120 or other branching unit 140. The branching units 140 may add or drop one or more other channels λ to/from the branch terminals 130. For example, a WDM signal 105 originating at the first trunk terminal 110 may include information occupying one or more channels λ. Likewise, a WDM signal 105 originating at the branch terminal 130 may occupy one or more channels λ. Both WDM signals 105 may be transmitted to the branching unit 140 that passes certain channels λ therethrough from the originating, first trunk terminal 110 along the communication trunk 102 without interruption to the second trunk terminal 120. The branching unit 140 may be configured to drop, i.e., extract information from, one or more channels λ originating from the first trunk terminal 110 and pass the dropped channels λ to the branch terminal 130. The branching unit 140 may also be configured to add, i.e., insert information on, certain channels λ originating from branch terminal 130 to the WDM signal 105 originating from the first trunk terminal 110 and pass the resulting WDM signal 105 (that includes the added information) onto the second trunk terminal 120. In some examples, the WDM signal 105 originating from the first trunk terminal 110 is fully terminated at branching unit 140, in which case only the added information from branch terminal 130 would be passed onto the second trunk terminal 120. Other branching units 140 may similarly pass through, add, and/or drop certain channels λ.

Any branching unit 140 may be disposed in an undersea environment and may be seated on the ocean floor. Additionally or alternatively, the branching unit 140 may be in a terrestrial environment and may be co-located at the same central office as the branch terminal 130. The communication trunk 102 may thus span between beach landings, or may provide a terrestrial connection between two terminal stations.

Each cable segment 102 may include one or more sections of fiber optic cable including optical fiber pairs and one or more repeaters 150 to provide a transmission path for bi-directional communication of optical signals 105 between the first and second trunk terminals 110, 120. The system 100 may be configured as a long-haul system, e.g. having a length between at least two of the terminals 110, 120, 130 of more than about 600 km, and may span a body of water, e.g., an ocean.

The repeater(s) 150 may include any optical amplifier/repeater configuration that compensates for signal attenuation on the transmission path. For example, one or more repeaters 150 may be configured as an optical amplifier, such as an erbium doped fiber amplifier, a Raman amplifier, or a hybrid Raman/EDFA amplifier. Also, one or more repeaters 150 may have an optical-electrical-optical configuration that regenerates an optical signal by converting it to an electrical signal, processing the electrical signal and then retransmitting the optical signal. A system bandwidth may coincide with the usable bandwidth of the optical amplifiers within the system 100.

Multiple terminals/stations 110, 120, 130 share optical bandwidth of the same fiber pair 102 by separating the whole spectrum into bands B using optical filters in the OADMs 140. A band B includes two or more wavelengths λ (also referred to as channels) residing spectrally adjacent to one another. By adding/dropping one or more bands B of signal wavelengths λ at each OADM 140, only signals 105 having wavelengths λ adjacent to the spectral edges E of the band B are affected by asymmetry penalties and high loss. The term “spectral edge” refers to the wavelength λ contained within a band B of wavelengths λ that is immediately adjacent to a wavelength λ not included within that particular band B of wavelengths λ. None of the signals 105 having wavelengths λ within the added/dropped band experience this spectral distortion.

Referring to FIG. 2, in some implementations, the OADM 140 has a guard band B_(G), which is a group of guard channels or wavelengths λ_(G) that resides between information bearing wavelengths λ_(I). A wavelength λ designated as a guard channel λ_(G) is not relied on to carry information and is considered a sacrificial wavelength λ. In other words, guard bands B_(G) are used to protect information bearing wavelengths λ_(I) residing in adjacent bands B, such as express wavelengths λ_(E) or added wavelengths L_(A), from spectral distortion while traversing an OADM 140. Locating guard band B_(G) between pass-through or express bands B_(E) of channels λ and added/dropped bands B_(A) of channels λ protects pass-through and added/dropped channels λ from asymmetry and high loss penalties by allowing the guard band B_(G) to absorb those penalties.

Referring to FIGS. 2 and 3, using a guard band B_(G) to separate spectrum, however, results in the loss of valuable spectrum. Submarine cables 102 typically have a total spectrum of 4 THz and guard bands B_(G) typically have a width of 200-300 GHz, resulting in at least 5% of lost spectrum to each guard bands B_(G). To eliminate this diminishing affect, the OADM(s) 140 may filter the optical signals 105 so that adjacent bands B overlap by a certain number of channels λ in an overlap band B_(O), rather than being separated by a guard band B_(G). This allows for the full use of the spectrum. The overlap band B_(O) of channels/wavelengths λ includes common wavelengths λ between a spectral edge E, E₁ of the first band B_(E) of wavelengths λ and a spectral edge E₂ of the second band B_(A) of wavelengths λ, and guard band B_(G) with sacrificed channels λ_(G). Without overlapping spectrum (the overlap band B_(O)), there is a stranded portion of the spectrum (i.e., the guard bands B_(G)) between pass bands or between a pass band and a drop band. By using the overlapping spectrum, it is possible to eliminate the stranded spectrum.

Overlapping spectrum allows adjustment of an add/drop ratio at an add/drop site. This can be particularly useful for submarine wet plant systems where the OADMs 140 (also known as Branching Units) are deployed in water and may remain for 25-30 years and cannot be re-adjusted based on traffic pattern changes. The overlapping spectrum architecture enables the ability to adjust the add/drop ratio to respond to unpredictable or unforeseen changes in traffic demand. For example, without overlapping spectrum, the add/drop ratio is fixed at a certain value, e.g., X % at a certain OADM 140. This means that X % of the traffic is dropped at that site and (100−X) % is expressed through. If the traffic mix of drop vs. express changes in the future, it is not feasible to support that new traffic mix without overlapping spectrum. With overlapping spectrum, however, it is possible to adjust the add/drop ratio by making changes at the end points (i.e., the stations 110, 120, 130) that are always accessible while keeping the wet plant (submerged portion, such as the branching units (OADMs) 140) the same over the life of the communication trunk 102 (e.g., a submarine cable).

Overlapping spectrum may result in collision of common channels λ from two terminals/stations 110, 120, 130, if both terminals/stations 110, 120, 130 send channels λ in the overlapping band B_(O) of the spectrum. To prevent collisions, one terminal 110, 120, 130 of a fiber pair may have priority over another terminal 110, 120, 130 for using overlap channels λ_(O) of the overlap band B_(O).

Referring to FIGS. 1 and 3, in some implementations, a system rule determines usage of the overlap channels λ_(O). For example, channels/wavelengths λ from the first trunk terminal 110 to the second trunk terminal 120 or from the branch terminal 130 to the second trunk terminal 120 may use the overlap channels λ_(O). Additionally or alternatively, the overlap channels λ_(O) may be reserved for express channels/wavelengths λ_(E) of an express band B_(E) between the first and second trunk terminals 110, 120. Different rules may apply, and the terminals 110, 120, 130 may agree to different rules for various conditions.

The overlap band B_(O) of wavelengths λ, which includes common wavelengths λ between a spectral edge E, E₁ of the express band B_(E) and a spectral edge E, E₂ of the so add/drop band B_(A) can be used for wavelengths λ of either the express band B_(E) or the add/drop band B_(A). Moreover, the overlap band B_(O) may be variable. In other words, in some implementations, the overlap band B_(O) is not fixed, but rather is variable in size.

Tables 1 and 2 below illustrate at least some of the differences between: (a) non-overlapping filters and (b) overlapping filters. Referring to Table 1, a non-overlapping spectrum architecture does not have the benefit of a variable add/drop ratio. As a result, the express band B_(E) has a baseline fraction of X, the guard band BG has a baseline fraction of Y, and the add/drop band B_(A) has a baseline fraction of 1−(X+Y), as illustrated in FIG. 2.

TABLE 1 Non-Overlapping Spectrum Overlap Express Guard Add/Drop Fraction Band Band Band Baseline Fraction 0 (zero) X Y 1 − (X + Y) Variable Add/Drop Ratio NA NA NA NA

Referring to Table 2, an overlapping spectrum architecture does have the benefit of a variable add/drop ratio. As a result, the express band B_(E) has a baseline fraction of X, the add/drop band B_(A) has a baseline fraction of (1−X), and the overlap band B_(O) has a baseline fraction of Z, as illustrated in FIG. 3. The guard band Be is no longer used and therefore has a baseline fraction of zero. The variable add/drop ratio for the express band B_(E) is X±Z, and the variable add/drop ratio for the add/drop band B_(A) is (1−X)±Z.

TABLE 2 Non-Overlapping Spectrum Overlap Express Add/Drop Fraction Band Band Baseline Fraction Z X 1 − X Variable Add/Drop Ratio X ± Z (1 − X) ± Z

The OADM 140 may include one or more tunable/adjustable filters 142 that produce the overlap band B_(O). The overlap band B_(O) may have a set/standard size or variable size to fit different applications. In each case, the terminals 110, 120, 130 may agree on which terminal 110, 120, 130 uses the overlap channels λ_(O) of the overlap band B_(O) for all, some, or particular transmissions of optical signals 105. One or more of the terminals 110, 120, 130 and/or a spectrum manager 160 may manage the overlap band B_(O). In some examples, the spectrum manager 160 tunes the filter(s) 142 of the OADM(s) 140 to provide a fixed or variable sized overlap band B_(O). Moreover, the spectrum manager 160 may manage usage of the overlap band B_(O) by the terminals 110, 120, 130, by setting and/or enforcing rules of overlap band usage. If the traffic mix of drop vs. express changes, the spectrum manager 160 may alter the size of the overlap band B_(O) and/or assign a portion of the overlap band B_(O) to express wavelengths λ_(E) of the express band B_(E) and a remaining portion of the overlap band B_(O) to add/drop wavelengths λ_(A) of the add/drop band B_(A).

The shape of the filter 142 can be any arbitrary combination of a pass band and a null band. While the filter shape shown in the figures is illustrative, the filter 142 can be any arbitrary shape to define the express band B_(E) and the add/drop band B_(A) of the spectrum. It does not have to be a single step function. Moreover, the overlap band B_(O) defined between the express band B_(E) and the add/drop band B_(A) can also be arbitrary.

While in some implementations, the OADM branching unit 140 assigns (e.g., at the direction of a terminal 110, 120, 130 or a spectrum manager 160) all of the overlap channels λ_(O) of the overlap band B_(O) to one terminal 110, 120, 130; in other implementations, the OADM branching unit 140 assigns (e.g., at the direction of a terminal 110, 120, 130 or a spectrum manager 160) a number of overlap channels λ_(O) of the overlap band B_(O) to one terminal 110, 120, 130 and a remaining number of overlap channels λ_(O) of the overlap band B_(O) to another terminal 110, 120, 130. The OADM branching unit 140 may split the overlap band B_(O) evenly or unevenly between two terminals 110, 120, 130.

To implement optical add-drop multiplexing in the branching unit 140, the branching unit 140 may implement three functions: splitting, filtering and combining. With regard to the splitting function, optical power on one input fiber to the configuration is split into two or more outgoing fibers. An optical coupler is one example of a device that can implement the splitting function. Filtering involves blocking/transmitting portion of input optical spectrum from one or more outgoing fibers. An attenuator and an all-pass filter are examples of filter configurations that do not discriminate by optical wavelength. Optical filters that transmit or block one or more specific wavelength bands can be implemented using technologies known to those of ordinary skill in the art, e.g., thin films and fiber Bragg gratings. The combining function involves merging optical signals from two or more sources onto a single output fiber. An optical coupler is one example of a device that can implement the combining function.

In some examples, the OADM branching unit 140 includes three filter types: a band pass filter drop (BPF-D) 142 d, a band pass filter add (BPF-A) 142 a, and a band reuse filter (BRF) 142 r. In the event that the optical cable segments 102 coupled to the branching unit 140 is repeaterless. BRF-A 142 a and BRF-D 142 d may be optional. The filters may have fixed or reconfigurable transmittance characteristics.

Referring to FIGS. 4 and 5, in some implementations, the concept of using an overlap band B_(O) versus a guard band B_(G) to merge, rather than separate bands B can be applied to spectrums that include conventional (C) and long (L) bands of wavelengths λ. An optical system 400 may include a splitter 410 that splits a signal 105 including both C and L band wavelengths λ_(C), λ_(L) in half (50/50 split) into first and second sub-signals 105 a, 105 b. A first filter 420 filters out (e.g., a drop filter drops) L band wavelengths λ_(L) from the first sub-signal 105 a, and a second filter 430 filters out (e.g., a drop filter drops) C band wavelengths λ_(C) from the second sub-signal 105 b. The resulting first sub-signal 105 a includes only C band wavelengths λ_(C), and the second sub-signal 105 b includes only L band wavelengths λ_(L). The optical system 400 may have a rule that an overlapping band B_(O) of C and L band wavelengths λ_(C), λ_(L) is dedicated to one band of wavelengths λ_(C), λ_(L) or another or fractionally divided between the two bands of wavelengths λ_(C), λ_(L). In the example shown, the optical system 400 divides the overlap band B_(O) equally, 50/50, between the C and L band wavelengths λ_(C), λ_(L). As such, both the C channels and the L channels benefit from the overlap band B_(O) equally. The overlap B_(O) eliminates any spectral gap between the C-bands and the L-bands, and thus avoids any loss of spectrum incurred by spectral gaps.

FIG. 6 is a schematic view of an exemplary arrangement of operations for a method 600 of optical communication that includes filtering 602 a first band B_(E), C of wavelengths λ_(E), λ_(C) of a communication spectrum for a first communication segment 105 a and filtering 604 a second band B_(A), L of wavelengths λ_(A), λ_(L) of the communication spectrum for a second communication segment 105 b. The second band B_(A), L of wavelengths λ_(A), λ_(L) overlaps the first band B_(E), C of wavelengths λ_(E), λ_(C) in an overlap band B_(O) of wavelengths λ_(O). The first band B_(E), C of wavelengths λ_(E), λ_(C) includes a first fraction of the overlap band B_(O) of wavelengths λ_(O) for the first communication segment 105 a and the second band B_(A), L of wavelengths λ_(A), λ_(L) includes a remaining fraction the overlap band B_(O) of wavelengths λ_(O) for the second communication segment 105 b.

In some implementations, the first band B_(E), C of wavelengths λ_(E), λ_(C) includes the entire overlap band B_(O) of wavelengths λ_(O) for the first communication segment 105 a and the second band B_(A), L of wavelengths λ_(A), λ_(L) excludes the overlap band B_(O) of wavelengths λ_(O) from the second communication segment 105 b. Other delegations of the overlap band B_(O) are possible as well.

The overlap band B_(O) of wavelengths λ_(O) may include common wavelengths λ between a spectral edge E of the first band B_(E), C of wavelengths λ_(E), λ_(C) and a spectral edge E of the second band B_(A), L of wavelengths λ_(A), λ_(L). In some examples, the first filter 142, 420 and/or the second filter 142, 430 provide a fixed sized overlap band B_(O) of wavelengths λ_(O) of the communication spectrum. In other examples, the first filter 142, 420 and/or the second filter 142, 430 are tunable to provide a variable sized overlap band B_(O) of wavelengths λ_(O) of the communication spectrum. The filtering may include adding, dropping, and/or reusing wavelengths λ.

In some implementations, the method includes receiving the first communication segment 105 a from a first trunk terminal 110, where the first communication segment 105 a is destined for a second trunk terminal 120 coupled by a communication trunk 102 with the first trunk terminal 110. The method also includes receiving the second communication segment 105 b from a branch terminal 130 coupled to the communication trunk 102. The overlap band B_(O) of wavelengths λ_(O) may be reserved for communications between the first and second trunk terminals 110, 120.

Various implementations of the systems and techniques described here can be realized in digital electronic and/or optical circuitry, integrated circuitry, specially designed ASICs (application specific integrated circuits), computer hardware, firmware, software, and/or combinations thereof. These various implementations can include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device.

These computer programs (also known as programs, software, software applications or code) include machine instructions for a programmable processor, and can be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the terms “machine-readable medium” and “computer-readable medium” refer to any computer program product, non-transitory computer readable medium, apparatus and/or device (e.g., magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions as a machine-readable signal. The term “machine-readable signal” refers to any signal used to provide machine instructions and/or data to a programmable processor.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, while the concepts disclosed herein are illustrated for submarine networks where the Branching Unit with OADM is not easily accessible and replaceable, this disclosure is applicable to non-subsea (i.e., terrestrial) networks as well. Moreover, the concept of flexible add/drop by using an overlap band B_(O) is extensible to dimensions other than spectrum sharing. Any other dimensions that have inherent orthogonality can be used for the flexible add/drop using an overlap band B_(O), such as time division multiplexing, space division multiplexing using multi core fibers or many mode fibers, polarization division multiplexing. Accordingly, other implementations are within the scope of the following claims. 

What is claimed is:
 1. An optical add-drop multiplexer comprising: a first filter filtering a first band of wavelengths of a communication spectrum for a first communication segment; and a second filter filtering a second band of wavelengths of the communication spectrum for a second communication segment, the second band of wavelengths overlapping the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band, the overlap band having a variable size; wherein the first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment.
 2. The optical add-drop multiplexer of claim 1, wherein the first band of wavelengths includes the entire overlap band of wavelengths for the first communication segment and the second band of wavelengths excludes the overlap band of wavelengths from the second communication segment.
 3. The optical add-drop multiplexer of claim 1, wherein the overlap band of wavelengths comprises common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths.
 4. The optical add-drop multiplexer of claim 1, wherein the first filter and/or the second filter provide a fixed sized overlap band of wavelengths of the communication spectrum.
 5. The optical add-drop multiplexer of claim 1, wherein the first filter and/or the second filter are adjustable to provide a variable sized overlap band of wavelengths of the communication spectrum.
 6. The optical add-drop multiplexer of claim 1, wherein filtering comprises adding, dropping, and/or reusing wavelengths.
 7. An optical system comprising: a first trunk terminal; a second trunk terminal; a communication trunk coupling the first trunk terminal to the second trunk terminal; and a branching unit disposed along the communication trunk and coupling a branch terminal to the communication trunk, the branching unit comprising an optical add-drop multiplexer comprising: a first filter filtering a first band of wavelengths of a communication spectrum for a first communication segment; and a second filter filtering a second band of wavelengths of the communication spectrum for a second communication segment, the second band of wavelengths overlapping the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band, the overlap band having a variable size; wherein the first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment.
 8. The optical system of claim 7, wherein the first band of wavelengths includes the entire overlap band of wavelengths for the first communication segment and the second band of wavelengths excludes the overlap band of wavelengths from the second communication segment.
 9. The optical system of claim 7, wherein the overlap band of wavelengths is reserved for communications between the first and second trunk terminals.
 10. The optical system of claim 7, wherein the overlap band of wavelengths comprises common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths.
 11. The optical system of claim 7, wherein the first filter and/or the second filter provide a fixed sized overlap band of wavelengths of the communication spectrum.
 12. The optical system of claim 7, wherein the first filter and/or the second filter are adjustable to provide a variable sized overlap band of wavelengths of the communication spectrum.
 13. The optical system of claim 7, wherein filtering comprises adding, dropping, and/or reusing wavelengths.
 14. The optical system of claim 7, wherein at least one of the first trunk terminal, the second trunk terminal, and the branching unit controls a size of the overlap band.
 15. The optical system of claim 7, further comprising a spectrum manager in communication with the first trunk terminal, the second trunk terminal, and the branching unit, the spectrum manager controlling a size of the overlap band.
 16. The optical system of claim 15, wherein the spectrum manager assigns a portion of the overlap band to wavelengths of the first communication segment and a remaining portion of the overlap band to the second communication segment.
 17. A method comprising: filtering a first band of wavelengths of a communication spectrum for a first communication segment; and filtering a second band of wavelengths of the communication spectrum for a second communication segment, the second band of wavelengths overlapping the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band, the overlap band having a variable size; wherein the first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment.
 18. The method of claim 17, further comprising altering a size of the overlap band.
 19. The method of claim 17, wherein the first band of wavelengths includes the entire overlap band of wavelengths for the first communication segment and the second band of wavelengths excludes the overlap band of wavelengths from the second communication segment.
 20. The method of claim 17, wherein the overlap band of wavelengths comprises common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths.
 21. The method of claim 17, further comprising assigning a portion of the overlap band to wavelengths of the first communication segment and a remaining portion of the overlap band to the second communication segment.
 22. The method of claim 17, wherein the filtering provides a fixed sized overlap band of wavelengths of the communication spectrum.
 23. The method of claim 17, wherein the filtering provides a variable sized overlap band of wavelengths of the communication spectrum.
 24. The method of claim 17, wherein filtering comprises adding, dropping, and/or reusing wavelengths.
 25. The method of claim 17, further comprising: receiving the first communication segment from a first trunk terminal, the first communication segment destined for a second trunk terminal coupled by a communication trunk with the first trunk terminal; receiving the second communication segment from a branch terminal coupled to the communication trunk;
 26. The method of claim 25, wherein the overlap band of wavelengths is reserved for communications between the first and second trunk terminals. 